{
  "bibcode": "2007GCN..6797....1B",
  "body": "D. N. Burrows (PSU), J. P. Osborne (U. Leicester), G. Chincarini \n(OAB), and N. Gehrels (NASA/GSFC) report on behalf of the Swift XRT team:\n\nThe Swift observatory continues to progress toward full operations. \nCurrently BAT and XRT are turned on and operating.  BAT is detecting \nGRBs and distributing Notices in real time (Barthelmy et al. GCN \n6781).  UVOT remains off and autonomous GRB slewing remains disabled \nuntil completion of the spacecraft gyro calibrations, which are still \non-going.  XRT is performing some delayed observations of GRBs and \nposting results by manually generated Circulars (such as GCN Circular \n6796).  XRT Notices are not yet enabled for automatic distribution.\n\nWe would like to issue the following general caveats for performance \nof the XRT following the observatory safe-hold recovery operations:\n\n1) Position determination: The safe-hold event resulted in all 3 \nSwift instruments being turned off and the observatory cooling down \nto survival temperatures.  The observatory is now operating at its \nnormal operating temperature once again, but there is a possibility \nthat the thermal cycle caused by the safe-hold may have resulted in a \nshift to the XRT boresight.  There is also a known boresight shift \nintroduced into the ACS software during the recovery operations as \npart of the change to a redundant gryo.  Initial indications suggest \nthat the XRT boresight shift is small, but until XRT boresight \nrecalibrations are complete, we cannot guarantee the same level of \naccuracy in XRT positions obtained prior to the \nsafehold.  Recalibration of the XRT boresight is our top priority and \nwe will provide updates on the results as they become available.  In \nthe meantime, we are increasing XRT position uncertainties (to 6-8 \narcseconds) to account for the uncertainty introduced by this event.\n\n2) Spectroscopy: the XRT team has been working for many months \ntowards a change in the CCD substrate voltage in order to reduce dark \ncurrent and minimize the effects of the elevated operating \ntemperatures of the XRT CCD.  This voltage change has now been \nimplemented and we are recalibrating the instrument.  Until the \nrecalibration is completed, there may be small gain changes (of order \n5%) and QE changes (of order 10% for E>6 keV) that are not correctly \naccounted for by the current CALDB files.  We will update the CALDB \nfiles as soon as possible.  Meanwhile, we urge caution in the \ninterpretation of spectral data, and we urge all users to process \ndata through xrtpipeline themselves using the latest available CALDB \nfiles until the changes have worked their way into the standard \nproduction pipelines at the Swift Data Center.  In particular, we \nnote that small gain errors can introduce spurious spectral features \nassociated with the Oxygen K edge, the Si K edge, and the Gold M edge \nin the instrument response.  These can be handled during spectral \nfits by allowing a gain adjustment in XSPEC to minimize these features.",
  "circularId": 6797,
  "createdOn": 1190113079000,
  "email": "dnburrows@gmail.com",
  "subject": "Swift Status and XRT Calibration Uncertainties",
  "submitter": "David Burrows at PSU/Swift  <dnburrows@gmail.com>"
}